For example, an automobile body is formed by assembling each of a plurality of panel members constituting the automobile body using a jointing means such as spot welding. In order to assemble the automobile body, the panel members become fixed to a conveyance truck by a clamping member and then a predetermined assembling operation such as spot welding is carried out at each work stage while the conveyance truck is moved in a body assembly line having work stages disposed per predetermined interval (e.g., see Japanese Patent Laid-open No. 4-283034. If a final stage and a first stage of the body assembly line are connected together by a returning line, the conveyance truck can be used in circulation.
The conveyance truck needs to be provided with the clamping member for fixing the panel members in their positioned states. When this clamping member is driven according to a movement of a piston rod of a pneumatic cylinder, a pipe that supplies air pressure to operate the pneumatic cylinder must be connected. However, during the movement of the conveyance truck, this pipe must be disconnected from the conveyance truck. Therefore, at the first stage and the last stage, the pipe is connected to the conveyance truck to supply the compressed air to the pneumatic cylinder, whereby an opening/closing operation of the clamping member is performed. However, while the conveyance truck moves in intermediate stages therebetween, the pipe is disconnected from the conveyance truck and, also during this movement, the panel members need to be continuously clamped.
Therefore, there is an air pressure cylinder provided with a braking mechanism so as to be able to control the piston rod even when the supply of air pressure is suspended. One example of this braking mechanism includes a locking mechanism, in which an engagement groove is formed in a side surface of a piston and, when the piston rod moves forward or backward to a predetermined position, a locking member is inserted into the engagement groove from a side direction of the cylinder using a spring force of a spring member in a direction orthogonal to a moving direction thereof, thereby locking a return movement of the piston rod.
Such a locking mechanism includes a type in which a slope surface such as a tapered surface is formed at a tip of the locking member in order to apply a thrust force to the piston rod and a spring force is applied to the locking member to apply the thrust force to the piston rod through the locking member. However, in order to downsize the fluid pressure cylinder having this locking mechanism, an important problem to be solved is that the spring member for applying the predetermined thrust force to the piston rod is downsized. Also in case of driving the locking member using fluid pressure such as compressed air, the fluid pressure cylinder can be downsized if the locking member can be driven by a piston with a small diameter.